As a one person Conservative caucus, he really will not have any impact on the legislature. Though if he is the start of the move to the Conservatives, then there is a chance for a four member caucus and the Conservatives become relevant.
What is much bigger deal is that the Liberals now only have 46 MLAs, this is still enough for a majority, but much narrower than before. If, as I think is likely, the Liberals will lose both by-elections, the standings in the legislature will be 46-39, the government only has a seven seat majority. If four more MLAs were to leave the Liberal caucus, the government would no longer have a majority.
I do not think we will see any wholesale rush for the door, but we could see some more resignations as MLAs. It takes more of those to happen for the government to lose its majority because the by-elections can be delayed for a long time.
I suspect John Martin will join John Van Dongen shortly in the Conservative caucus, but this still leaves them two seats short of party status in the house.
Meanwhile, I do have to wonder a bit about the motivation of John Van Dongen in making this move. His riding is one that the NDP would never win but could be won by the Conservatives. Was his move in part motivated by a desire to continue as MLA after the 2013 election? If he runs as a Conservative, he should be able to be re-elected. There are few places in BC where a sitting Liberal MLA could cross to the Conservatives and feel fairly safe in re-election in the next election. Someone like Ida Chong would gain no personal benefit from crossing the floor.
It was not so long ago that the Conservatives were really in the same political fringe like BC First or BC Refederation. Now with an MLA the Conservatives have relegated the assorted right wing fringe parties to irrelevancy in the next election.
In a final piece of trivia, the BC Conservatives last had an MLA 26 years ago when Graham Lea crossed the floor to them on March 27th 1986. Graham Lea had been an NDP MLA and cabinet minister and then run for the party leadership in 1984.